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Persistent Photoconductivity Used to Control Semiconductor Cells

Photonics SpectraAug 2017
RALEIGH, N.C. — Use of light to alter the conductivity of semiconductor material could provide a new way to control cell behavior on semiconductors used for bioelectronics. The approach draws on persistent photoconductivity, a phenomenon that causes some materials to become much more conductive when light is shined on them. When conductivity in these materials is elevated, the charge at the surface of the material increases. The escalation in surface charge can be used to direct cells to adhere to the material’s surface.

This image illustrates changes in photocurrent before and after exposure to UV light. Persistent photoconductivity is demonstrated even hours after the UV light has been turned off. This is illustrated by the pictograms showing charge carriers that come into contact with cells at the interface during in vitro experiments. Courtesy of Albena Ivanisevic.

Researchers at North Carolina State University used gallium nitride (GaN), a wide bandgap semiconductor that exhibits persistent photoconductivity, and PC12 cells, a line of model cells used in bioelectronics testing, to enable fabrication of a biointerface.

The researchers tested two identical groups of GaN substrates. One group was exposed to UV light, triggering its persistent photoconductivity properties, while the second group was not.

“There was a clear, quantitative difference between the two groups — more cells adhered to the materials that had been exposed to light,” said professor Albena Ivanisevic.

The conductivity of one template was altered through exposure to UV light and the behavior of PC12 cells was mapped under different substrate conductivity. The pattern size and roughness were combined with surface chemistry to change the adhesion of PC12 cells when the GaN was made more conductive after UV light exposure.

Further, during neurite outgrowth, surface chemistry and initial conductivity difference were used to facilitate the extension to smoother areas on the GaN surface. The researchers believe that these results could be used to develop novel bioelectronics interfaces to probe and control cellular behavior.

“There's a great deal of interest in being able to control cell behavior in relation to semiconductors — that’s the underlying idea behind bioelectronics,” said Ivanisevic. “Our work here effectively adds another tool to the toolbox for the development of new bioelectronic devices.

“We now need to explore how to engineer the topography and thickness of the semiconductor material in order to influence the persistent photoconductivity and roughness of the material. Ultimately, we want to provide better control of cell adhesion and behavior,” she said.

The conductivity increase exhibited by some nonmetallic materials, resulting from the free carriers generated when photon energy is absorbed in electronic transitions. The rate at which free carriers are generated, the mobility of the carriers, and the length of time they persist in conducting states (their lifetime) are some of the factors that determine the amount of conductivity change.